A significant proportion of newsprint is recycled. In a typical operation, the newsprint is pulped, screened to remove large containments, deinked, further screened to remove smaller containments, dewatered and then forwarded to stock make-up for a paper making machine.
During the screening operations, a variety of contaminants associated with the incoming newsprint are removed with generally larger and heavier contaminants being removed before smaller and lighter contaminants. For smooth and continuous operation of the recycling plant, it is essential that the screening operation further function efficiently. Unfortunately certain filters used in such operation are prone to plugging and screen wear.
Particulars problems have been encountered with a form of rotary screen filter which comprises a housing with which is fed the pulp for decontamination, a horizontal rotor which deems to circulate the pulp within the housing, and a circular accepts screen basket through which the decontaminated pulp is removed. The rejects fraction is slurried within the housing and is dumped from time to time.
The rotor is hollow and has external foils which assist both in circulation of the slurry within the housing and in expelling the accepts fraction through the screen basket. In proper operation, the screen feed is removed from the inlet by the foils past the screen basket through which the accepts fraction passes with the remainder moving past the rotor. The slurry is recirculated through the rotor interior and back for another pass. Such internal recycle is essential to ensure a proper separation of an accepts fraction from the rejects fraction.
However, heavy materials tend to build up on the interior surface of the rotor until eventually recirculation stops and the screen plugs. Attempting to close off the ends of the rotor and stop recirculation was not satisfactory, in that the coarse rejects particles simply rotate with the rotor foils and produce excessive wear of the screen.
A search in the facilities of the United States Patent and Trademark Office has located the following U.S. Pat. Nos. as the closest prior art:
1,134,304 PA0 2,621,793 PA0 4,238,324 PA0 4,287,055 PA0 4,316,768 PA0 4,697,982
This prior art describes a variety of screening devices for screening pulps but none addresses the plugging problem encountered with the horizontal hollow rotor screening device referred to above, nor do they suggest solutions to that problem.